1. Field of the Invention
The present invention relates to package structures and fabrication methods thereof, and more particularly, to an electromagnetic interference (EMI) shielding package structure and a fabrication method thereof.
2. Description of Related Art
For the purpose of fabrication of a semiconductor package, a semiconductor chip is electrically connected to a carrier such as a lead frame or a substrate, and an encapsulant made of epoxy resin is formed to encapsulate the semiconductor chip and the carrier, thereby protecting the semiconductor chip and the carrier against intrusion of external moisture or contaminants.
However, a semiconductor package in operation can easily be influenced by electromagnetic interference (EMI), thereby causing abnormal operation and poor electrical performance of the semiconductor package.
Accordingly, U.S. Pat. No. 5,166,772 discloses a structure with a metal shield embedded in the encapsulant thereof.
FIG. 1 is a cutaway view of the structure. Referring to FIG. 1, a chip 11 is disposed on a substrate 10 and electrically connected to the substrate 10 through a plurality of bonding wires 12, wherein the substrate 10 has at least a ground terminal 14, a perforated metal shield 13 is disposed to cover the chip 11 and electrically connected to the ground terminal 14, and an encapsulant 15 is formed to cover the metal shield 13, the chip 11, the bonding wires 12 and a portion of the substrate 10, thereby embedding the metal shield 13 in the encapsulant 15. The metal shield 13 shields the chip 11 from external EMI so as to improve electrical performance of the overall structure. Similar structures are also disclosed in U.S. Pat. No. 4,218,578, No. 4,838,475, No. 4,953,002 and No. 5,030,935.
However, since an additionally fabricated metal shield 13 is required in the above-described structure, the fabrication process of the structure is complicated. Further, the metal shield 13 that is required to cover the chip 11 and fixed to the substrate 10 increases the assembly difficulty. Furthermore, after the metal shield 13 is disposed on the substrate 10 to cover the chip 11, the encapsulant 15 must pass through the metal shield 13 for encapsulating the chip 11. Since the metal shield 13 is perforated, when the encapsulant 15 passes through the metal shield 13, turbulence can easily occur in the encapsulant, thus resulting in generation of air bubbles in the encapsulant and causing a popcorn effect in a subsequent thermal processing.
FIG. 2 is a cutaway view of a structure disclosed by U.S. Pat. No. 5,557,142. Referring to FIG. 2, a chip 21 is mounted on a substrate 20 and electrically connected to the substrate 20 through a plurality of bonding wires 22. Further, an encapsulant 23 is formed to encapsulate the chip 21, the bonding wires 22 and a portion of the substrate 20, and a metal layer 24 is formed on the exposed surface of the encapsulant 23 through coating or sputtering so as to shield the package structure from EMI. Similar structures are also disclosed in U.S. Pat. No. 5,220,489, No. 5,311,059 and No. 7,342,303.
Although the above conventional structure dispenses with complicated processes, the metal layer 24 must be formed by coating or sputtering after a singulation process, and it is difficult to perform component arrangement and pickup in a singulated package structure; hence, the above conventional structure is not suitable for mass production. In addition, the sputtering process cannot be applied in a package structure in which the encapsulant is flush with the sides of the substrate.
In a package structure disclosed by U.S. Pat. No. 7,030,469, a groove is formed on an encapsulant to expose bonding wires, and a conductive layer electrically connected to the bonding wires is formed in the groove and on the encapsulant, thereby achieving a shielding effect. However, the conductive layer must be made of a non-ferrous metal material and can only be formed on the groove and encapsulant by depositing or sputtering. Therefore, it cannot be applied in a package structure in which the encapsulant is flush with the sides of substrate. Further, the contact between the conductive layer and the bonding wires is point contact, which can easily result in poor electrical connection between the conductive layer and the bonding wires.
Therefore, it is imperative to overcome the above drawbacks of the prior art.